1) Field of the Invention
The present invention relates to the field of semiconductor fabrication and more particularly to metal-oxide-semiconductor (MOS) fabrication and to a process for preventing the formation of precipitates after a contact layer (e.g., a tungsten layer) etch back process.
2) Description of the Prior Art
To form semiconductor devices, it is often necessary to make contact to device regions through a dielectric layer. This is accomplished by first forming an opening or via (contact via) in the dielectric layer over the region to be contacted, and next filling the contact via with a conductive material.
In addition to filling the vias with a conductive material, it is necessary to electrically connect certain device regions with others as well as to provide for electrical connections to external leads. These requirements are met by forming a wiring layer on the surface of the substrate. The wiring layer is formed by depositing a conductive material on top of the dielectric layer in which the vias have been formed. The conducive layer is then masked and etched to leave continuous lines of conductive material which make the appropriate connections between device regions. These lines are called interconnects.
Several conductive materials can be used as contact via fill. In larger geometry devices, the via fill (plugs) and interconnects are formed in the same metallization step. For instance, aluminum (Al) can be deposited on the entire substrate, as well as in the vias. The areas over the vias and interconnects are then masked with photoresist and the aluminum is then etched from the remaining exposed areas, leaving the vias filled with aluminum as well as forming interconnect lines on the surface of the dielectric layer.
As semiconductor devices have become smaller, the aspect ratio (ratio of height to width) of the vias to the device regions has greatly increased. Aluminum deposition alone has proved to be inadequate in devices with high aspect ratios. The problems encountered include poor step coverage, poor contact integrity, and inadequate polarity. To overcome these shortcomings, tungsten and other refractory metals are being used as a contact filling for devices with submicron contacts before aluminum deposition and patterning. For example, a blanket tungsten layer is deposited on the substrate surface and fills the via. Next, the tungsten layer is blanket etched back to remove the deposited tungsten from the surface of the substrate, leaving a tungsten contact filling (or plug) in the contact openings (or vias). An aluminum layer is then deposited, coveting the substrate surface including the filled contact vias. This aluminum film is then patterned and etched to form the interconnect lines between devices.
In order to improve the reliability of the interconnects and contacts, it has become a widely accepted practice to deposit a barrier metal layer such as TiN, TiW, or the like within the contact hole using, for example, a sputter process. This barrier layer prevents undesirable reactions between the substrate material (e.g., Si) and the wiring material or between the dielectric layer (e.g., silicon oxide) or polysilicon and the wiring material (e.g., aluminum).
A problem with the current process of etching back a tungsten contact layer that is formed over a barrier layer such as a TiN layer (or any Ti containing layer) is that a precipitate often forms on the barrier layer or surface layer. This precipitate, often violet in color, can cause circuit failure by interfering with the metal and insulation layers formed over the TiN and precipitate. The precipitate can also cause reliability problems such as electro migration. In addition, the precipitate can cause metal peeling on a metal layer deposited over the precipitate.
Therefore, there is a need for a process to remove or prevent the formation of precipitates after tungsten etch back. This process optimally should be reliable, simple, fast, and inexpensive to implement.
Davis, U.S. Pat. No. 5,164,330 teaches tungsten etch back process for tungsten layers using a NF.sub.3 /Ar chemistry with three etch steps. This process reduces the amount of residue buildup in the etching reactor.
Kadomura, U.S. Pat. No. 5,227,337 discloses a two step tungsten etch back process where the first step uses S.sub.2 F.sub.2 gas at the high temperature and S.sub.2 F.sub.2 /H.sub.2 at a low temperature.
Petro et al. U.S. Pat. No. 5,326,723 teaches a method of cleaning a CVD process chamber used to deposit tungsten. The chamber undergoes an in-situ cleaning process with NF.sub.3 and H.sub.2 N.sub.2 plasmas.
Sumi, U.S. Pat. No. 5,254,498 discloses a method of forming a barrier metal structure in a contact hole to ensure good metal coverage by the metal. The invention forms an oxide layer over a metal barrier layer (e.g., W) in a contact hole and forming a contact metal over the oxide.
Woo, U.S. Pat. No. 4,833,099 teaches a N.sub.2 anneal after a tungsten deposition but before forming an oxidation layer over the tungsten layer. The N.sub.2 anneal inhibits the tungsten from reacting with oxygen in the oxidation step and allows the formation of a planar and uncontaminated oxide layer.